Rule 1

Use a period at the end of a complete sentence that is a statement.

I know that you would never break my trust intentionally.

Rule 2

If the last word in the sentence ends in a period, do not follow it with another period.

I know that M.D. She is my sister-in-law.

Please shop, cook, etc. I will do the laundry.

Rule 3

Use the period after an indirect question.

He asked where his suitcase was.